A fluidized bed is a suspension of solid particles in a stream of gas or liquid of sufficient velocity to support the particle by flow forces against the downward force of gravity Fluidized beds are critical components of important petrochemical processing units such as the catalytic cracking ("cat-cracking") of petroleum on catalytic particles to produce lighter and more valuable products as well as thermal cracking of heavy feeds on coke particles ("fluid bed cokers" or "flexi-cokers") to again produce lighter and more valuable feeds In cat-cracking the regenerator where coke is burned off the catalyst to produce "fresh catalyst" contains a fluidized bed The particles in the fluidized bed within the regenerator are approximately 60 micron diameter pellets of a zeolite. In the case of fluid bed-coking or flexi-coking, fluidized beds can be found in the heater, reactor and in the case of flexi-coking, the gasifier. The particles in this case are approximately 100 to 150 micron particles of coke.
Other fluidized beds containing small solids suspended in a gas include advanced coal combustion units where small particles of coal are suspended and burned to produce heat with minimum pollution and maximum efficiency. Yet another example is found in separation processes in the chemical industry where a fine suspension of particles is suspended in a flowing liquid. In general, fluidized beds are used in many large scale processes where it is desired to maximize the interaction between the surface of a particle and a surrounding gas or liquid.
Fluidized beds can contain volume mass densities for the case of fluid bed cokers and regenerators of the order of 40 pounds per cubic foot and particle velocities of several feet per second. Fluidized beds of the order of 10 to 50 feet in diameter are found in coking and cat cracking. With bed heights of the order of 10 to 60 feet the contained fluids range from less than a hundred to more than a thousand tons of particles. "Gas Fluidization Technology" by Geldart (Wiley 1986) is a review of the technology.
In many cases in the petrochemical industry, the material within the fluidized bed is at an elevated temperature Thus the vessel will often be lined by several inches of refractory. Direct access to the contained particles is thus very difficult. This difficulty is compounded when heavy petroleum feed is being injected into the fluidized bed as in the case of cokers and cat-crackers. Under these circumstances the interior of the fluidized bed can foul any window or probe if special precautions are not taken. Monitoring the flow condition in the fluidized bed is usually limited to static pressure measurements and inferred flow data derived from such measurements. Special precautions to avoid fouling include use of an inert gas to maintain openings in such static pressure measurements and eliminating the pressure drop of the inert gas through a "pressure bridge". While pressure measurements can suggest flow maldistribution within the vessel containing the fluidized bed, they rarely (if at all) pinpoint the region of flow maldistribution.
Flow maldistribution within a fluidized bed can arise from a variety of causes. One example is where the bed is "slumped" in one region of the reactor, greatly reducing the efficiency of the chemical process going on within the fluidized bed. Under conditions of "bed slump", fluidization gas can be channeled to the other side of the reactor, leading to regions of flow turbulence. Under such circumstances excessive attrition of the particles within the fluidized bed can occur leading to an excessive number of fines emitted into the gaseous product of the reactor or into the atmosphere.
Another example where flow maldistribution is a problem is in fluid bed cokers where flow blockage in the region where fluidization steam enters the reactor can lead to a buildup of particle agglomeration and introduce a bed "bogging" condition. Under such circumstances until the flow blockage is eliminated, feed cannot be injected into the coker. Again while pressure or temperature measurements may be useful for identifying the poor flow state of the unit they are seldom useful in identifying the region of the coker reactor on either a vertical or horizontal plane where such a condition exists.
Another example is in the steam stripping section of a catalytic cracking unit (cat-cracker) where hydrocarbon residues on the catalyst are stripped before the spent catalyst is sent to the regenerator for burning off of the coke left from the cracking reaction Efficient stripping has a direct effect on yield of the unit. Under certain circumstances, either flow blockages within the stripping region or large differences in steam input across the stripping region can lead to low efficiency of the stripping reaction. Again it would be desirable to determine the region where a flow maldistribution or blockage is located in order to eliminate it.
Another example is where a critical element of the flow distribution within a fluidized bed is damaged. For example, in many cases flow within the bed is dominated by gas and particle flow through a grid forming the bottom of the dense phase of the bed. Under special circumstances holes in the grid can become blocked, leading to regions of bed slump. Other possibilities include damage to the support structure of the grid due to the large forces exerted by gas pressure across the blocked grid. Under these circumstances a large quantity of a chemically reacting gas may bubble through the bed destroying the uniformity of the process and leading to serious problems in the temperature distribution across the fluidized bed.
In considering all of the above examples where it is desirable to locate the region of flow maldistribution or anisotropy within a fluidized bed it is important to note that it is often equally important to an operator to know that the flow within the fluidized bed is uniform and isotropic. With this information in hand the operator can direct his energies toward other causes of flow, yield, attrition or other problems indicated by global instrumentation such as pressure, temperature or excessive "fines" emitted in the process either into the atmosphere or into the product.
However, if flow maldistribution is occurring within a fluidized bed in a specific region there are a variety of corrective actions that can be taken to remedy the flow maldistribution problem. Steam lancing can be directed toward the region of blockage to clear obstructions. Stripping steam flow can be realigned to produce flow uniformity. In the case of excessive flow of reacting gas through a damaged grid support, "torch oil" can be injected into the unit to reduce gas flow by combustion and hence reduce excessive temperature gradients in regions of the reactor where such gradients would damage components such as cyclones In the most extreme case where a region of the unit has been identified as exhibiting a consistent pattern of flow maldistribution, baffles can be inserted during unit shutdown to produce flow uniformity.
There is a major need to measure the state properties of a fluidized bed non-intrusively from the exterior without penetrating the wall of the vessel and where the traditional use of static pressure gauges is inadequate. The non-intrusive determination of local bed mass density, P.sub.M, particle mass, M, and particle velocity, V., and what can be inferred from them as to the flow state of the fluidized bed would be of great value to the operators of such units, in maintaining design performance, improving product yield and trouble shooting poor flow or fluidization conditions within the fluidized bed.
While pressure, temperature and net volume or mass flow are the normal way of monitoring the state of fluidization within a fluidized bed or while a unit is operating, there are a variety of techniques that can be brought to bear on functioning fluidized beds. One example is the use of gamma rays or neutrons to determine the mass density of particles within the vessel. This technique can only be used if the walls and/or diameter of the vessel are less than a critical value since the technique is based on deriving the density from absorption. Too large a vessel diameter, or too thick a wall drops the detected signal below the level of background noise and the mass density cannot be determined. In addition the presence of intense radioactive sources and the necessity to construct elaborate structure to support the detectors of the radiation reduce the use of this technology to elaborate field tests or where major uncertainties arise over the operation of the fluidized beds. The gamma or neutron technique is expensive, has to be scheduled in advance and usually beyond the capability of normal refinery personnel.
Non-intrusive probes that can be used to monitor the flow state of experimental fluidized beds would also be of great value in complementing visual, radiographic and radioactive tracer studies of flow in order to improve or modify existing designs, or for pilot plant studies A current review of a wide variety of electrical, optical, thermal and mechanical technology for studying the hydrodynamics of experimental gas-solid fluidized beds is contained in a recent review by N. P. Cheremisonoff (Ind Eng. Chem Process Dev. 25, 329-351 (1986)). The r.RTM.view presents techniques that are "best suited for laboratory-scale systems, [although]adaption to industrial pilot facilities and/or commercial units is possible in some cases". However, examination of the presented techniques suggest they suffer from the usual disadvantages of being intrusive, easily contaminated by the process or as in the case of so many of the radioactive techniques severely restricted by environmental or safety considerations.
In the July 1985 issue of the Journal of the American Society of Lubrication Engineers (Lubrication Engineering), J. W. Spencer and D. M. Stevens (of Babcock & Wilcox, a McDermott company of Lynchburg, Va.) describe a technique for "detecting and characterizing particulate matter in fluid flow systems" by using "acoustic emission technology". In this technology the impact of particles generates high frequency surface vibrational waves which are detected as "pulses" by resonant piezo electric transducers. As described in the article, only sensors in contact with probes inserted into the flowing stream correlated with bulk quantity or size of particles in the stream. Sensors mounted non-intrusively on the walls of the pipe "did not correlate well with probe-mounted transducers. Again, this technique is intrusive since it requires penetration of the walls of the vessel (see also U.S. Pat. Nos. 3,816,773 and 4,095,474 which describe similarly intrusive techniques).
A review of the prior art shows that there are no known technologies for reliably and safely measuring or inferring the flow state of two phase flow within a fluidized bed that meet the following criteria:
(1) Non-intrusive and hence requiring neither penetration of the wall or the constructing of external frame works to support radioactive sources and detectors and hence permitting trouble shooting of commercial units;
(2) Non-radioactive and/or suitable for "on-line" monitoring of fluidized beds or transfer lines on working commercial units;
(3) Capable of applying in a "non-intrusive manner" to the refractory lined vessels and transfer lines containing solid particles in the presence of gases such as air, steam and/or volatile hydrocarbons with wall temperatures as high as 250.degree. to 500.degree. C.